The Sudden Close-Up
macOS heavily integrates its software with the Multi-Touch trackpad. In the Safari web browser, there is a feature called “Smart Zoom.” By default, if you double-tap (not click, just a light tap) the trackpad with two fingers, Safari attempts to automatically identify the main block of text or image you are looking at and instantly zooms in to fill the screen with it. While occasionally useful for reading small text, it is highly prone to accidental activation. A slight graze of your fingers while scrolling can suddenly magnify a random sidebar ad, completely disorienting you and interrupting your reading.
Disabling the Smart Zoom Gesture
To ensure a stable browsing experience and stop macOS from automatically zooming in Safari when you accidentally brush the trackpad, you must disable the specific Multi-Touch gesture in your system settings.
Click the Apple logo in the top-left corner of your screen and select System Settings (or System Preferences on older versions). Scroll down the left-hand navigation sidebar and click on Trackpad (if you are using a Magic Mouse, click Mouse instead). In the main Trackpad settings window, click on the Scroll & Zoom tab at the top. Look for the setting labelled Smart zoom (the description will say “Double-tap with two fingers”). Click the toggle switch next to it to turn it Off (or uncheck the box). You can now safely rest your fingers on the trackpad without Safari randomly magnifying the webpage.